The present invention relates to apparatus and methods for determining the polarization of electromagnetic signals.
Electromagnetic signals such as radio waves and light have a property referred to as polarization. Radar operates by transmitting an electromagnetic signal to a target and comparing the signal reflected from the target with the transmitted signal. In modern electronic warfare, targets avoid detection from enemy radar by using various countermeasures such as, jamming an enemy radar signal impinging on the target with a signal denying range information to the enemy and creating false reflected signals to deceive the enemy radar system. To be effective, the signals created by the countermeasure system should have characteristics such as polarization corresponding to the signal characteristics expected by the enemy system as, for example, characteristics of the return signals expected by an enemy radar system. In some cases, the enemy radar may change its signal polarization rapidly. Such a radar system is referred to as xe2x80x9cpolarization agile.xe2x80x9d If the enemy radar is polarization agile, the countermeasure system must be capable of determining the polarization of the transmitted signal rapidly, so that the countermeasure system can change the signals which it emits. For example, a jamming system carried on an aircraft and intended to defeat a polarization agile enemy radar system should determine the polarization of the incoming radar signal and alter the polarization of the jamming signal accordingly. If the jamming system does not do this, the jamming signal will not match the polarization of the return signals from the aircraft. The enemy radar receiver can reject the jamming signals and acquire meaningful return signals. Delay in measuring the incoming signal polarization can allow the enemy system to acquire meaningful return signals for a sufficient time to find the position of the aircraft. Conversely, where a radar or communications system must overcome enemy jamming, it is desirable to measure the polarization of the jamming signal and transmit the radar or communications signal with a different polarization.
However, traditional polarization measuring techniques do not provide polarization measurements rapidly enough to counteract a polarization agile enemy system. Just as the receiving system becomes accustomed to one polarization, the enemy system changes polarization.
At a given point in space along the path of an electromagnetic wave and at a given instant in time, an electric field points in a particular direction, denoted by a vector, {right arrow over (E)}. This vector is perpendicular to the direction of travel of the signal or xe2x80x9cpropagation vector.xe2x80x9d The polarization of an electromagnetic wave is described by the orientation of the electric field vector and the manner in which this vector varies with time.
The polarization vector can be split into components Ex and Ey along orthogonal x and y axes perpendicular to the direction of travel of the electromagnetic wave. The component along the x axis commonly is referred to as the xe2x80x9chorizontalxe2x80x9d component, whereas the component along the y axis is referred to as the xe2x80x9cverticalxe2x80x9d component. Although these terms are used herein, it should be appreciated that these directions may be arbitrary directions unrelated to the normal gravitational frame of reference. At any given point in space, Ex and Ey vary with time. For example, for a sinusoidal wave having frequency xcfx89, Ex=Asin(xcfx89t) and Ey=Bsin((xcfx89t)+xcex1), where t is time, xcex1 is a phase difference and A and B are the magnitudes of the Ex and Ey components. When the Ex and Ey components are in phase (xcex1=0), the electric field is linearly polarized. In this condition, the electric field vector at a given point always lies on the same plane. When the Ex and Ey components are out of phase (xcex1xe2x89xa00), elliptical polarization results. When the Ex and Ey components of an elliptically polarized electromagnetic signal are of equal magnitude (A=B) and are 90xc2x0 or 270xc2x0 out of phase, the signal is said to be circularly polarized.
To measure signal polarization, a dual-aperture (polarized) antenna and a device known as a polarimeter are required. The dual-aperture antenna provides one electrical signal Vh representing the Ex or horizontal component of the electric field of a signal impinging on the antenna, and another electrical signal Vv representing the Ey or vertical component of the electric field of the same signal. These signals typically are amplified and filtered separately in a dual-channel receiver before passing to the polarimeter. The polarimeter compares these signals to determine their relative magnitudes and the phase difference between them.
A prior art analog polarimeter is shown in FIG. 1. The horizontal signal Vh is supplied to one input of a four port directional coupler 200 of a type referred to as a xe2x80x9chybrid.xe2x80x9d The vertical signal Vv is supplied to the input of a phase shifter 202 which applies a known phase shift xcex3 to that signal. The phase-shifted signal is supplied to another input of the directional coupler 200. The coupler 200 couples the signals supplied to its input and provides a signal at a first output 204 representing the coupled power output or sum of the input signals supplied to the circuit, and also provides another signal at a second output 206 representing another combination of the input signals with a specific phase shift, 180xc2x0 in this example, between the input signals. The first or sum output 204 of circuit 200 is supplied to the input of a further phase shifter 208 which applies a known phase shift xcfx86. The output of this phase shifter is connected to one input of another directional coupler 210, which is similar to the first 200. The second or difference output 206 of coupler 200 is connected directly to the other input of coupler 210. Thus, when time-varying Vv and Vh signals are applied to the polarimeter, one time-varying output signal, referred to as the xcexa3 signal appears at the difference output 212 of coupler 210. Another time-varying output signal referred to as the xcex94 signal, appears at the sum output 214 of coupler 210. The output signals are supplied to a dual-channel receiver and logarithmic amplifier 216 which monitors the amplitudes of these signals and provides a signal representing a ratio between their amplitudes. This ratio signal is supplied to a null adaptive tracker 218, which adjusts the phase differences xcfx86 and xcex3 applied by the phase shifters to achieve a null condition as discussed below.
The relationships between the xcexa3 and xcex94 output signals appearing at the outputs 212 and 214 of the second 180xc2x0 coupler 210 and the input signals Vv and Vh are referred to as the xe2x80x9ctransfer functionsxe2x80x9d of the polarimeter. These transfer functions depend on the phase shift values xcex3 and xcfx86 applied by the phase shifters 202 and 208. Conversely, there is a relationship between the transfer functions which yield output signals with particular characteristics and the phases and amplitudes of Vv and Vh. Stated another way, there is a relationship between the phase shifts xcfx86 and xcex3 which yield particular output signal characteristics and the phases and amplitudes of the input signals Vv and Vh.
In particular, for the components illustrated in FIG. 1, the ratio       "LeftBracketingBar"    Δ    "RightBracketingBar"        "LeftBracketingBar"    Σ    "RightBracketingBar"  
between the amplitude |xcex94| of the xcex94 output signal and the amplitude |xcexa3| of the xcexa3 output signal will be at a minimum or null condition when:                               γ          =                      2            ⁢                          xe2x80x83                        ⁢                                          tan                                  -                  1                                            ⁡                              (                                  b                  a                                )                                                    ,                  xe2x80x83                ⁢        and                            (        1        )                                φ        =                                            3              ⁢              π                        2                    -                      α            .                                              (        2        )            
Where:
a is the amplitude of the horizontal component Vh; b is the amplitude of the vertical component Vv; and xcex1 is the phase difference between these components.
Solving for the amplitude ratio   b  a
and phase difference xcex1 from the xcex3 and xcfx86 values,                                           b            a                    =                      tan            ⁡                          (                              γ                2                            )                                      ,                  xe2x80x83                ⁢        and                            (        3        )                                α        =                                            3              ⁢              π                        2                    -                      φ            .                                              (        4        )            
Thus, the parameters that characterize the polarization of the input signal, such as the amplitude ratio   b  a
and the phase difference xcex1 between the components of the signal can be found from the phase shifter values xcfx86 and xcex3 that yield the null condition or minimum ratio             "LeftBracketingBar"      Δ      "RightBracketingBar"              "LeftBracketingBar"      Σ      "RightBracketingBar"        .
Tilt angle, xcfx84, of an elliptically polarized signal is also derivable from the polarimeter phase shifter values xcfx86 and xcex3 at the null condition as                     τ        =                              1            2                    ⁢                      xe2x80x83                    ⁢                                                    tan                                  -                  1                                            ⁡                              [                                  tan                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          2                      ⁢                      γ                                        )                                    ⁢                                      xe2x80x83                                    ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          φ                      -                                                                        3                          ⁢                          π                                                2                                                              )                                                  ]                                      .                                              (        5        )            
In operation, tracker 218 sets the xcex3 phase shifter 202 to hold xcex3 constant at an arbitrary value and adjusts the xcfx86 phase shifter 208 to vary xcfx86 in an iterative or trial-and-error process until the output ratio       "LeftBracketingBar"    Δ    "RightBracketingBar"        "LeftBracketingBar"    Σ    "RightBracketingBar"  
is at a minimum for the arbitrary value of xcex3. The tracker 218 then holds xcfx86 constant and adjusts phase shifter 202 to vary xcex3 in a further iterative process until the true minimum or null condition is found.
Other analog polarimeters use different networks, typically including phase shifters and couplers. However, the overall principle of operation is the same. The transfer function or functions of the polarimeter is adjusted iteratively to yield output signals having predetermined characteristics, and the polarization of the signal is found from the transfer function or functions which yield those characteristics. Polarimeters of this type can provide accurate measurements of signal polarization. However, they require considerable time to perform the required iterations.
One aspect of the invention provides apparatus for measuring the polarization of a received signal that has two orthogonal components. Apparatus according to this aspect of the invention preferably includes first and second system input terminals where the components of the received signal are applied, and a plurality of channels, each such channel being simultaneously connected to the input terminals so that each channel will be supplied with both of the components. Each channel includes means for combining the received signal components according to a combining scheme having one or more characteristic variables so as to produce a set of one or more output signals. Most preferably, the combining means of each channel includes one or more phase shifters, and the characteristic variables include values of the phase shifts applied by the phase shifters of the various channels. In one embodiment, the combining means of each channel includes a combining circuit including two phase shifters and two couplers similar to those used in the iterative polarimeter discussed above, except that the phase shifters of each channel operate at preset phase shift values which do not change. Each such combining circuit produces a xcex94 output signal and a xcexa3 output signal.
Different ones of the combining means apply combining schemes with different values of the characteristic variables, as, for example, different phase shifts. Thus, different values of the characteristic variables are associated with the different channels and with the different sets of output signals produced by these channels. The apparatus further includes means for measuring at least one parameter of each said set of output signals as, for example, one or more detectors for measuring the ratio of amplitudes of the output signals or a function of such ratio. The apparatus further includes means for estimating the polarization of the received signal based on said measured parameters of a plurality of the sets of output signals and the values of the characteristic variables of the combining schemes associated with such sets of output signals.
In one arrangement, the estimate of polarization is made by selecting a particular set of parameters which best meets a preset criterion. In the example where the set of parameters measured for the set of output signals for each channel consists of the amplitude ratio, the means for estimating the polarization may include analog or digital components which select the lowest amplitude ratio and thus identify the channel which provided the set of output signals having the lowest amplitude ratio. In this example, different preset values of the phase shifts are associated with each channel, and selection of a particular channel implicitly selects values of the phase shifts which are closest to the values which would produce the null condition.
In another arrangement, the means for estimating polarization includes means for deriving a theoretical set of characteristic variable values which would yield a set of output signals meeting one or more predetermined criteria if applied to the components of the received signal, this derivation being based on the measured parameters of the output signals from the various channels and the values of the characteristic variables associated with the measured parameters. In the example discussed above, for a given incoming signal the amplitude ratio is a dependent variable of two independent variables, namely, the values of the phase shifts. The measured values of the amplitude ratio constitute sample values taken at known values of the independent variables. The means for deriving may include a programmed processor performing a mathematical minimization algorithm. This approach provides particularly accurate estimates of polarization with a limited number of channels.
Another aspect of the invention provides methods of determining the polarization of a received signal that has two orthogonal components. Methods according to this aspect of the invention desirably include combining the components with one another according to a plurality of different combining schemes, each having different values of characteristic variables to produce a plurality of output signals. A set of one or more of said output signals is created by each of the combining schemes. Two or more of different combining schemes are applied simultaneously to produce two or more sets of output signals simultaneously, as, for example, by routing the components of the received signals through plural parallel channels as discussed above in connection with the apparatus. The method further includes measuring one or more parameters of each said set of output signals, and providing an estimate of the incoming signal polarization based on the measured parameters of the various sets of output signals. This step may be performed, for example, by selecting one said set of output signals which best meets one or more predetermined criteria and thereby selecting the combining scheme associated with that set. Alternatively or additionally, this step can be performed by deriving a set of characteristic variables as discussed above in connection with the apparatus.
Apparatus and methods according to certain preferred embodiments of the invention can provide extremely rapid estimates of the signal polarization. Certain preferred embodiments of the invention can be applied to measurement of polarization of microwave signals as, for example, signals in the range of 4-18 GHz.
Other objects and advantages of the apparatus and methods will become apparent to those skilled in the art after reading the detailed description of the preferred embodiment.